Pleonasm is the use of more word

Word

In language, a word is the smallest free form that may be uttered in isolation with semantic or pragmatic content . This contrasts with a morpheme, which is the smallest unit of meaning but will not necessarily stand on its own...

s or word-parts

Morpheme

In linguistics, a morpheme is the smallest semantically meaningful unit in a language. The field of study dedicated to morphemes is called morphology. A morpheme is not identical to a word, and the principal difference between the two is that a morpheme may or may not stand alone, whereas a word,...

 than is necessary for clear expression: examples are black darkness, or burning fire. Such redundancy

Redundancy (language)

In linguistics, redundancy is the construction of a phrase that presents some idea using more information, often via multiple means, than is necessary for one to be able understand the idea....

 is, by traditional rhetorical criteria

Rhetoric

Rhetoric is the art of discourse, an art that aims to improve the facility of speakers or writers who attempt to inform, persuade, or motivate particular audiences in specific situations. As a subject of formal study and a productive civic practice, rhetoric has played a central role in the Western...

, a manifestation of tautology

Tautology (rhetoric)

Tautology is an unnecessary or unessential repetition of meaning, using different and dissimilar words that effectively say the same thing...

.
Often, pleonasm is understood to mean a word or phrase which is useless, cliché

Cliché

A cliché or cliche is an expression, idea, or element of an artistic work which has been overused to the point of losing its original meaning or effect, especially when at some earlier time it was considered meaningful or novel. In phraseology, the term has taken on a more technical meaning,...

d, or repetitive, but a pleonasm can also be simply an unremarkable use of idiom

Idiom

Idiom is an expression, word, or phrase that has a figurative meaning that is comprehended in regard to a common use of that expression that is separate from the literal meaning or definition of the words of which it is made...

. It can even aid in achieving a specific linguistic effect, be it social, poetic, or literary.